Introduction to the Functions and Control of the Alimentary Tract

Question 1

What are the digestive functions of the stomach?

Answer 1

1. Accomodation and storage
2. Mechnical and enzymatic breakdown
3. Slow delivery of chyme to the duodenum

Question 2

Describe where the storage of digestive products takes place in the body.

Answer 2

1) STOMACH:
Food may be stored here during the first stage of digestion; it may remain there for around an hour, unmixed (where it acts as a reservoir). The fundus and the body of the stomach (with thinner muscle tone) relax, allowing a large volume of food storage (about 1.5L). The vagal reflex inhibits smooth muscle tone.
The antral region mixes and grinds food with gastric secretions (digestion).

2) COLON/RECTUM:
Here, the storage of indigestive residues and faecal matter occur.

Question 3

Describe the storage of gastric secretions.

Answer 3

The stomach stores 2-3 litres of gastric juice ever 24 hours (made up of mucous, pepsinogen, intrinsic factor, lipase, etc.) which helps in the digestion and absorption of food.

Some examples of gastric secretions:

• MUCOUS: secreted by goblet cells and mucous neck cells, acts as a lubricant by acting as a barrier that protects the stomach and colon, especially from gastric acid (prevents trauma)
• LIPASE: converts triglycerides to fatty acids and glycerol
• PEPSIN: secreted by chief cells or peptic cells as pepsinogen, helps in protein digestion
• HCL: secreted by parietal cells, important for defence of the gut
• INTRINSIC FACTOR: secreted by parietal cells, helps with Vitamin B12 absorption

Question 4

Describe the pancreatic secretions into the gut.

Answer 4

These are often called ‘local hormones’. They are secreted from cells in the mucosa, but, unlike hormones, the chemical acts locally on adjacent cells via the interstitial fluid.

An exmaple would be somatostatin, which inhibits gastric release in the stomach.

Question 5

List some exocrine secretions that aid in digestion.

Answer 5

SALIVARY GLANDS: mucous (lubrication for mastication and speech); lipase

GASTRIC GLANDS: HCl, pepsin, mucous

PANCREAS: bicarbonate ions, enzymes (eg. amylase, lipase, carboxypeptidase)

LIVER: bile salts, bile acids

Secretions from numerous glands with ducts enter the lumen of the gut and are involved in digestion, lubrication and protection.

Question 6

List some endocrine secretions that aid in digestion.

Answer 6

Secretions called ‘hormones’ are synthesised by ductless glands, and enter the blood stream, travel to their target tissue(s) where they bind to specific receptors to elicit their effects.

Examples include:

• GASTRIN: stomach (G-cells in antrum)
• SECRETIN: duodenal mucosa
• PANCREOZYMIN-CHOLECYSTOKININ: duodenal mucosa
• INSULIN: pancreas (β-cells)

Exocrine, endocrine and paracrine secretions allow for active digestion and the control of digestion and gastric motility (and energy homeostasis).

Question 7

What is the point of absorption?

Answer 7

For food to be of use to the body, the nutrients resulting from digestion must be transported across the intestinal epithelium into the blood (eg. glucose, amino acids, etc.) or lymph via lacteals (fats/lipids).

Absorption occurs mainly in the small intestine. The absorption of fluid occurs in the small intestine and colon. The colon absorbes 90% of water, reducing the volume to 200 ml of semi-solid faecal matter. Disorders of fluid secretion and absorption are important (together with motility) in the pathogenesis of diarrhoea.

Question 8

What is motility?

Answer 8

It is the movements of the muscular wall (which is mostly smooth muscle except extreme ends of the upper oesophagus/rectum) which allow:

• movement from one region to another (law of gut); mass evacuation
• mechanical degradation (eg. gastric antrum)
• mixing lumen contents (eg. small intestines)
• transport of nutrients, water and of urea and electrolytes

Question 9

What are the different methods of excretion?

Answer 9

Drugs and some products of normal metabolism may leave the body in:

• saliva
• bile
• faeces
• (vomit)

Indigestible food residues (eg. tomato skin) leave the body in the faeces.

Question 10

How does the gut contribute to defence in the body?

Answer 10

Like the skin and airways, the gut epithelium is an interface with the ‘contaminated’ outisde world (it is exposed to the external environment).

The intestine is the largest mucosal surface in the body and is probably exposed to the heaviest burden of environmental antigens.

Like the skin, if there is a breach in the barrier, ‘toxins’ will enter the blood. It is also the largest lymphoepithelial organ.

Question 11

What different defence mechanisms does the gut have?

Answer 11

The gut is unsterile as it is open to the external environment, and the following help to protect the gut:

1. sight, smell and taste alerts us to harmful food substances
2. vomit reflex
3. acid in the stomach (HCl) kills most harmful bacteria
4. mucous secretions
5. netural bacterial flora prevents the colonisation of harmful bacteria
6. aggregation of lymphois tissue (eg. Peyer’s patches) are able to mount a response to food-borne atnigens - analyse and response to pathogenic microbes

Peyer’s patches are locates in the lamina propria layer of the mucosa and extend into the submucosa of the ileum.

Question 12

Describe the metabolic function of the gut.

Answer 12

The liver is a major metabolic organ in the abdominal cabity and weighs about 1.3 kg in an adult.
It is involved in carbohydrate, nitrogen and lipoprotein metabolism as well as the production of bile and excretion of bilirubin.

Question 13

The gut is autonomously innervated.

List what the parasympathetic and sympathetic systems stimulate and inhibit.

Answer 13

PARASYMPATHETIC:
- stimulates salivation

SYMPATHETIC:

• inhibits salivation
• relaxes the bladder
• contracts the rectum

Question 14

Describe the innervation involved in the relaxation of the fundus.

Answer 14

Relaxation of the reservoir (fundus) is mediated by reflexes and can be differentiated into 3 types:

• RECEPTIVE (mechanical stimulation of the pharynx - mechanoreceptors, sight)
• ADAPTIVE (vagal innervation, {NO,VIP}, tension of the stomach)
• FEEDBACK (nutrients, CCK)

The receptive, adaptive and feedback-relaxation of the stomach are mediated by non-adrenergic, non-cholinergic (NANC) mechanisms (ie. inhibition involving NO, VIP, etc.) as well as by reflex chains involving the release of noradrenaline. When the stomach is ready to receive food, very early on, noradrenaline is released from the sympathetic nerve fibres, which help the stomach to relax.

Question 15

What is PACAP?

Answer 15

PACAP is pituitary adenylate cyclase (AC)-activating peptide (from the secretin family of peptides). It is isolated from the pituitary and has been shown to stimulate AC activity in the anterior pituitary.
There are high levels in the brain, but it is also found in the gut (in the myenteric and submucosal ganglia). It mediates the neuronal regulation of gastric acid secretion (contributing to intestine motility).

It stimulates the relaxation of colonic cmooth muscle and stimulates pancreatic secretion (it stimulates insulin and glucagon secretion in humans).

Question 16

Describe the steps of food movement through the stomach.

Answer 16

1) Ripples of contraction move the food towards the antrum (thicker muscle layer).
2) The pyloric sphincter is often relaxed, but closes upon the arrival of the peristaltic wave.
3) The repulsion of chyme causes the opening of the pyloric sphincter.
4) Small, partially digested material is squirted through the pyloric sphincter into the duodenum.
5) Repulsion of antral contents nackwards towards the body allow for mixing/grinding.

The sieving effect means that viscous and solif metter are retained in the stomach.

Question 17

What are the different types of control of the alimentary tract?

Answer 17

• endocrine
• paracrine
• neural (includes the vago-vagal reflex and the enteric/local reflex)
• metabolic (type of food eaten)

Question 18

Describe the endocrine control of the alimentary tract.

Answer 18

Hormones are carried in the blood from their site of production to their target site. All hormones produced by the gut are peptides.

When the stomach is distended, neurons are stimulated, releasing GRP, which acts on G cells to release gastrin. The gastrin travels through the blood and affects two cell types: enterochromaffin-like cells (EMF cells) and parietal cells. ECL cells release histamine, which promotes acid secretion from parietal cells.
Acetylcholine also affects ECLs and parietal cells in a similar manner.

The gut can also be influenced by hormones produced elsewhere in the body (eg. oestradiol, progesterone).

Question 19

Describe the paracrine control of the alimentary tract.

Answer 19

Note that hormones (endocrine) go to the target tissue via the blood, but paracrine agents go via the interstitial fluids.

An example of paracrine control would somatostatin.
Increased H+ stimulates D cells, which release somatostatin. This then acts on G cells to inhibit the release of gastrin.

Question 20

Describe the neural control of the alimentary tract.

Answer 20

VAGO-VAGAL REFLEX:
It is described as a type of reflex in which both the afferent (‘sensory’) and efferent (‘motor’) axons are in the vagus nerve trunk. It is a reflex circuit within the GIT. The pathway is via the brain stem (medulla). it describes the reflex control of responses to gut stimuli via the NTS and DMVN.

ENTERIC NERVOUS SYSTEM/ LOCAL REFLEX:
Two nerve fibres are intrinsic to the gut:
1. Myenteric Plexus (Auerbach’s plexus): motor function (P + S)
2. Submucosal Plexus (Meissner’s plexus): intestinal secretions (only P)

Auerbach’s plexus has both parasympathetic and sympathetic input, whereas the Meissner’s plexus has only parasympathetic fibres and provides secretomotor innervation to the mucosa nearest the lumen of the gut.

They reflexly regulate GI functions entirely within the wall of the gut. It is connected to the CNS via the parasympathetic and the sympathetic fibres but can function autonomously without these connections.

These effects are mediated entirely by the enteric nervous system (the third component of the ANS).

Question 21

Give an example of how the innervation of the gut recognises food in the system.

Answer 21

When there is food in the oesophagus, it will stimulate mechanoreceptors, which will send impulses to the brain and back via the vagal afferent (and efferent) fibres.

Question 22

What is the colo-colonic reflex?

Answer 22

It is when the presence of food or food products/distension of the stomach increases the motility of the colon in response - again, it is due to the stretch in the stomach and the by-products of digestion in the small intestine.

Question 23

How does stimulation of the autonomic nervous system affect colon motility?

Answer 23

An increase in parasympathetic activity will increase contraction in the proximal colon.
An increase in sympathetic activity will decrease colon movement.

Question 24

How does stimulation of the autonomic nervous system affect Auerbach’s plexus?

Answer 24

An increase in parasympathetic (cholinergic) activity increases gastric motility and secretion, while an increase in sympathetic (adrenergic) activity will result in decreased gastric motility and secretion.

Question 25

Describe the metabolic control of the alimentary tract.

Answer 25

The rate of emptying is dependent upon the material’s ability to be absorbed.

Carbohydrates are emptied quickly into the duodenum.
Proteins are slow-emptying, while fats are even slower.

Fatty acids in the duodenum lead to a decrease in gastric emptying by increasing the contractility of the pyloric sphincter.

Question 26

Describe the peristalsis of the alimentary tract.

Answer 26

It is a wave of propulsive contractions that moves contents of the gut towards the anus. It is slower in the large intestine when compared to the small intestine.

Distention initiates the contraction, while vagal inhibitory and excitatory fibres control movement.

(With the peristaltic waves passing over the stomach, some luminal material is forced into the duodenum.)